Hot stuff 04 August 2016

Last month, Federal Mogul unveiled its technology strategy for heavy-duty engine pistons and associated components for an age of waste heat recovery. John Kendall reports from Munich

In the wake of Euro 6, it is clear that EU legislators’ attention will primarily be focussed on reducing CO2 emissions and hence also fuel consumption. That chimes well with vehicle manufacturers, which have been engaged on engine downsizing programmes for the past 25 years. Indeed, few modern road-going engines are now larger than 13 litres.

Such moves have helped reduce fuel consumption and have also resulted in engines typically working harder. That’s good for SCR (selective catalytic reduction) aftertreatment efficiency, but it also means engine cooling requirements are greater than ever before.

The industry needs to step up to the plate here. Why? Because beyond vehicle aerodynamics and low rolling resistance tyres – both expected to play part in further CO2 reductions – the strongest focus will be on further improving driveline efficiency. That and recovering waste heat energy, which can then be fed back into the driveline.

So there are implications for both engine and engine component designers such as Federal Mogul, which majors on everything from pistons to piston rings, valves, valve seats, cylinder liners, bearings and seals. And that, in turn, is why the company’s recent unveiling of its technology strategy for waste heat recovery is so interesting.

Federal Mogul’s Envirokool heavy-duty diesel steel piston will get its public debut at the IAA Hanover commercial vehicle Show in September. What’s new is a sealed-for-life cooling chamber inside the piston crown filled with purpose-specified cooling oil and an inert gas, designed to prevent the coolant from oxidising. Friction welding is used to integrate the cooling chamber with the piston. After the chamber has been charged with the oil and gas mix, it is sealed permanently with a welded plug.

The cooling chamber enables the piston crown to withstand combustion temperatures more than 100°C greater than today’s maximum. As a result, carbon deposits that would normally form on the cooling gallery can be burnt off, ensuring that the latter continues to dissipate heat for the life of the piston.

The principal cooling method for the cooling gallery is a standard oil cooling jet, directed at the underside of the piston, using lubricant from the engine system. However, Federal Mogul claims that, because the cooling gallery becomes so effective, the cooling oil jet flow can be reduced by around 50%. That also reduces parasitic losses, while also de-coupling the engine oil from cooling.

“This technology is mainly applicable to engines with waste heat recovery,” explains Norbert Schneider, director of global application engineering at Federal Mogul. Why? He makes the point that simply making a piston hotter doesn’t in and of itself save fuel. “Right now, the piston is cooled tremendously with lubrication oil. All that energy is then dissipated by the radiator so is totally wasted.

“The idea here is to increase the temperature of the exhaust gas by cooling the piston much less. But then the piston gets hotter, which is why we need this technology. However, with much hotter exhaust gases, [engine designers] can recover a significant proportion of this [much greater] waste heat.”

Received wisdom suggests that up to 5% of energy can be recovered from waste heat systems and fed back to the drivetrain. The favoured method appears to be a heat pump, converting the heat into mechanical energy. “Future legislation will demand a significant reductions in fuel consumption for truck engines and right now this [approach] is seen as having serious potential,” says Schneider.

However, raising exhaust gas temperature will present other challenges for engine designers as it will also lead to increased formation of NOx emissions, currently controlled by EGR (exhaust gas recirculation) and/or SCR aftertreatment systems.

“It will be several years before we can expect Envirokool to go into production,” agrees Scheider, “This is a relatively early stage. We have been carrying out testing for two years with more than a thousand hours of successful results so we are now in a pre-development stage with a small number of engine companies. We would expect mass production to begin from 2021.”

New valve materials

Engine downsizing has also been the trigger for new inlet and exhaust valve materials for heavy-duty diesels. With 10- and 11-litre engine now capable of producing around 500bhp, valves that can withstand higher temperatures and combustion pressures of 220bar and above have become necessary. Greater levels of EGR have also exposed valves to increased risk of wet corrosion.

Additionally, engine braking systems such as Jacobs’ Jake Brake add mechanical loading to valves. And yet the pressure to reduce the cost of components continues to grow. These factors have triggered research into new materials for valves, valve seats and valve guides.

Focusing first on cost, high nickel content is a key issue, as Gian Maria Olivetti, chief technology officer at Federal Mogul Powertrain, explains. “For demanding applications the conventional answer is a premium material such as our ECMS-Ni80A, but with a nickel content greater than 70% this is not always an economic solution. So we have established new ways to make more effective use of the expensive alloying elements such as nickel – by validating materials with equivalent hot strength to ECMS-Ni80A, but lower alloy content.”

By researching alternative combinations of materials, such as nickel (Ni), chromium (Cr) and manganese (Mn), suitable new metals can be formulated, he explains. For its part, Federal Mogul has developed an upgraded alloy, which the company calls ECMS-2512NbN, developed from standard CrMn heat-treated steel.

This material has been designed for use in both inlet and exhaust valves that experience higher temperatures. Standard steels contain around 3% nickel, but by increasing this content to around 12% and fine-tuning the combination of the other alloy components, it is possible to improve both hot strength and corrosion resistance, says Olivetti.

Meanwhile, where higher temperature performance is a requirement, Federal Mogul claims that its ECMS-Ni36 high-performance steel alloy can offer better resistance to hot oxidation than its industry standard equivalent ECMS-3015D – a heat-treated steel with 15% Cr and 31% Ni. Despite comprising only 36% nickel, this alloy offers similar tensile strength to the superalloy ECMS-Ni80A, which consists of more than 70% nickel, according to the company.

“The very high level of alloying elements present makes these materials quite special,” comments Guido Bayard, director pf global valve train technology for Federal-Mogul Powertrain. “In order to predict reliability and conformity to engine applications, a wide range of rig and laboratory tests on wear, corrosion and durability have been conducted, with a series of engine application trials on dynamometers also completed.”

In fact, Federal-Mogul has already started series production of valves made from ECMS-2512NbN and ECMS-3015D. Trials are underway with unnamed customers, using ECMS-Ni36 ahead of other production launches. Both ECMS-2512NbN and ECMS-Ni36 are suitable for valves in 10-16-litre engines in a range of applications, including on and off-highway.

That said, similar challenges for valve seat inserts and valve guides have also triggered developments with new powder metal materials. Federal Mogul claims these too can contribute to reduced CO2 emissions by enabling gas flow across the valve seat chamfers to be more efficient.